As described in detail in U.S. Pat. No. 5,312,455, modern, multi-programmable, multi-mode implantable pacemakers, and by extension other implantable cardiac stimulators, monitors or the like, are equipped with sense amplifiers that are designed to detect depolarizations of myocardial tissue constituting features of the electrogram (EGM) as a "sense event" and record data related to the sense event and/or affect the operation of such devices. The time varying and oscillating EGM amplitude reflects the depolarizations of atrial and ventricular chamber heart tissue in a normal activation sequence, wherein the oscillations of the signal are characterized by convention as a "PQRST" complex. The PQRST complex is described below in reference to the illustration thereof in FIG. 5 in reference to sensing electrodes located in atrial and ventricular locations. The sense electrode(s) location affects the shape of the PQRST complex and polarity of the successive oscillations, but usually the individual transitions are evident.
In practice, regardless of its shape, the PQRST complex is conventionally simplified by reference to the same polarity oscillations which constitute the relatively low amplitude "P-wave", followed by a higher amplitude "R-wave" (separated by an "A-V" interval) and then concluded by a low amplitude "T-wave". The intrinsic "escape interval" between successive P-waves or R-waves defines the heart rate (in beats per minute). The P-wave has a pulsatile wave shape manifesting the electrical depolarization wave of the atria, and the R-wave and T-wave shapes manifest the rapid depolarization and slow re-polarization waves of the ventricles, respectively.
In some applications, related to monitoring of the patient's heart activity, it is desirable to sense and record the full PQRST EGM or to providing responsive device-delivered therapy sensing and recording other electrical noise and interference. However, it is also desirable to detect the P-wave and/or the R-wave and so that the A-V and intrinsic escape intervals can be timed in both monitors and therapeutic cardiac stimulators and so that certain operations may be initiated on detection of the P-wave or R-wave. In this process, it is undesirable to detect and mistake the T-wave for the P-wave or for a succeeding R-wave, for example, or to miss a P or T-wave in the noise. The simplest thing to do is to establish blanking periods as described in U.S. Pat. No. 4,379,459 to Stein which retains information about noise through the blanking period.
Naturally, the amplitude, and to some extent the rate of change or slope, of the P-wave and R-wave signal may be used to distinguish one from the other and from the T-wave. With respect to amplitude distinction, the sense amplifiers of modern monitors and/or stimulators are configured to have a bandpass response and a "sensitivity" to the signal amplitude that may be varied in order to trigger a "sense event" when the proper P-wave or R-wave is present. The greater the sensitivity, the lower the signal amplitude necessary to trigger the sense event. The sensitivity is typically a function of the gain of the sense amplifier or a function of a "sensing threshold" against which the amplified signal is compared. In the latter case, the sensitivity is therefore inversely related to the sensing threshold.
In practice, the detection of P-waves and R-waves is complicated by a number of factors or abnormal conditions which mask, elevate or diminish the signal amplitudes, so that the signals, even if present may not be sensed (referred to as "undersensing") or too many sense events may be triggered (referred to as "oversensing"). Tissue overgrowth of implantable electrode surfaces, referred to as "aging", can chronically alter the signal amplitude. Short term electromagnetic interference (EMI) conducted through the body from external sources and muscle artifacts or myopotentials originating within the body may mask the EGM. The EMI sources can either be intentionally introduced by such events as programmer downlink, cautery, defibrillation, and fluoroscopy, or unintentionally introduced by any number of radiating electronic devices, such as cell phones and anti-theft detection devices. Exercise levels, heavy breathing, medications and temperature changes may also constitute an abnormal condition that influences the EGM signal level. (In some cases, the changing impedance of the heart can drown out the impedance signals used to find minute ventilation as in U.S. Pat. No. 5,271,395 (Whalstrand et al.), requiring some kind of low pass filtering to find the breath signal. Impedance measurement may use the same sense amp (amplifier) that finds the EGM.)
When oversensing occurs in a demand pacemaker, the pacing function may be erroneously continuously inhibited, leading to loss of cardiac output and injury to the patient. An early way of countering oversensing due the continuous presence of EMI, was to sense the noise and "revert" to asynchronous pacing, i.e., go into "reversion". This kind of reversion is also called "noise reversion pacing."
However at least since the Gobeli & Adams patent (U.S. Pat. No. 3,927,677) and many which follow it, reversion in response to EMI has been supported with schemes to cancel the EMI noise as in for example Meltzer, U.S. Pat. No. 5,647,379. Nevertheless such circuits and their operation in the presence of continuous EMI are still referred to as "reversion" circuits or operations, even though demand pacing is assured.
For many years, it has also been possible to remotely program the sensitivities of such sense amplifiers in implanted pacemakers, cardioverter/defibrillators, other stimulators, or monitors within a range in order to compensate for other, more long term, changes in signal amplitude. In practice, in one approach, a range of programmable sensing threshold values is provided for selection by a physician at implantation and which can be periodically re-programmed during patient follow-up visits to ensure that the proper P-wave or R-wave is being detected.
In addition, automatic sensitivity adjusting systems have been proposed and implemented in both external and implantable embodiments of such stimulators or monitors. A first approach, similar to the reversion approach described above, adjusts the threshold to the average signal level over a certain time span, as shown, for example, in U.S. Pat. No. 4,240,442. In a somewhat related fashion, threshold levels to comparator inputs are adjusted to as shown in U.S. Pat. Nos. 4,768,511; 5,312,455; and 5,395,393. In these approaches, constant gain sense amplifier stages first amplify the filtered EGM and then compare the amplified signal to the adjustable threshold level and generate a sense event if the threshold is exceeded.
In further approaches, adjustable gain sense amplifier stages are employed, and automatic sense amplifier gain adjustment is employed as proposed in U.S. Pat. Nos. 4,000,461; 4,325,384; 4,708,144 and 4,880,004. Generally, comparators are employed in AGC feedback networks to make the gain adjustment in relation to a target voltage.
In certain of these approaches, e.g. the above-referenced '384, '004, and '461 patents, one or two additional comparators and further peak thresholds are employed to provide a peak target or range in which to bracket the peak of the sensed signal. The sensing threshold and the peak thresholds are adjusted in tandem to ensure that the peak amplitude either approaches and "dithers" about a single peak threshold or falls between a pair of peak thresholds.
An enhancement to these was shown in Baker et al., U.S. Pat. No. 5,103,819, which, after a fashion, adjusted the sense amp gain up in the presence of senses below a low threshold level and up in the presence of senses above a high threshold. Many patients who have implanted cardiac pacemakers, cardioverter/defibrillators or arrhythmia (or other) monitors are susceptible to a variety of arrhythmias. For example, many such patients have irregular heart rhythms marked by the occurrence of ectopic origin or circus rhythm conducted P-waves and ectopic origin R-waves. Typically, these premature atrial contractions (PACs) or premature ventricular contractions (PVCs) manifest excessive amplitudes and widths which can skew the adaptive threshold values. Runs of such PVCs and PACs may be processed in the above-described systems to reduce sensitivity, resulting in loss of sensing of succeeding normal conducted depolarizations.
These and other patients may be also be susceptible to episodes of atrial or ventricular tachyarrythmias with widened signal wave shapes and decreased amplitude. Such tachyarrythmia episodes of diminished peak amplitude may be processed in the above-described systems to increase sensitivity, resulting in oversensing of succeeding normal conducted depolarizations and any other signals.
In addition, abnormal noise and myopotential episodes occur from time to time as described above, which, if not accounted for can cause the sensing threshold to be unduly elevated when the noise abates, resulting in undersensing.
Finally, in actual experience with cardiac pacemakers, there may be long periods of little intrinsic cardiac activity to sense because the underlying heart rate is lower than either a fixed lower pacing rate or the pacing rate is being adjusted to the need for cardiac output as indicated by a physiologic or patient activity sensor of a rate responsive pacemaker. Such episodes may be entirely normal but constitute an abnormal condition to the sensitivity adjusting algorithm resulting in the sensitivity being increased to a level resulting in oversensing of noise within the sensing intervals.
Therefore, a need continues to exist for automatic sensitivity adjustment of a cardiac sense amplifier that takes into account short term variations in cardiac signal amplitude due to respiration, posture changes, exercise, drug therapies, etc., avoids responding to noise and myopotentials, premature beats or other arrhythmias, and compensates for periods of little sensing which thereby minimizes instances of oversensing and undersensing resulting from inappropriately adjusted sensing thresholds. Each patent referred to in this section is hereby by this reference incorporated into this application in their respective entireties so as to include all their disclosure without the need for reiteration.